In non-geostationary satellite communication systems, earth stations must transmit signals to mobile or hand-held communication devices (also known as user terminals) via satellite on a regular basis for synchronization, paging, and short message delivery. The mobile or hand-held communication devices, also known as user terminals, each include a receiver to facilitate communication with non-geostationary satellites. Mobile or hand-held communication devices may be equipped with transmitters as well as receivers depending on the capability of the system.
One limiting constraint placed on hand-held communication devices employed in non-geostationary satellite communication systems is that they must be turned on in order to receive any signals. Therefore, each hand-held communication device should have a sleep/wake cycle with short wake intervals between long sleep intervals in order to conserve battery power.
To ensure that each hand-held communication device has a sleep/wake cycle with short wake intervals between long sleep intervals, the hand-held communication devices, which generally are grouped together, are divided into subgroups. Each hand-held communication device associated with a particular subgroup wakes at specified intervals. If a hand-held communication device assigned to a particular subgroup wakes only during specific time intervals, and there are a large number of subgroups, the wake time of the hand-held communication device will be short relative to its sleep time. When a hand-held communication device has a long sleep time relative to its waking time, battery power in the device may be conserved.
Tiedemann, Jr. et al., U.S. Pat. No. 5,392,287, which is incorporated herein by reference, teaches a system for reducing receiver power consumption in battery-powered communication devices having a transmitter and one or more receivers. The system assigns each receiver to a particular time slot during which the device will enter an active state so that it can receive or transmit messages. When the battery powered communication device is not in an active state, it may perform a non-communication related function, or conserve battery power. The communication device disclosed by Tiedemann, Jr. et al. may be periodically or continuously synchronized.
Davis et al., U.S. Pat. No. 5,392,457, which also is incorporated herein by reference, also teaches a battery saving scheme in which the receiving functions of a battery powered communication receiver are supplied selectively with power so that the receiver may receive a coded message signal.
Murai et al., U.S. Pat. No. 5,274,843, which also is incorporated herein by reference, teaches a paging apparatus having a battery saving function. The paging apparatus is periodically turned on in synchronization with the detection of synchronization signals and is turned off in response to the detection of a specific code.
When a hand-held communication device is awake, it can receive transmission bursts from the non-geostationary satellites. The bursts transmitted by each non-geostationary satellite serve two functions simultaneously: synchronization and information delivery. There are two situations when synchronization signals are needed. In the first situation, a hand-held communication device which is not synchronized to the system must acquire transmissions when the device is initially provided with power without knowing when a burst is going to arrive.
In a second situation in which synchronization signals are needed, a hand-held communication device that is synchronized to the system knows approximately the time slots when bursts may be arriving. However, because of clock drifts within the hand-held communication device, the device requires synchronization updates every few minutes.
In order to provide sufficient power for operation within a building, high penetration paging signals are transmitted from the non-geostationary satellites one at a time. In any given time interval, one beam is selected and a high power signal is transmitted in that beam. As the beams move across the surface of the earth, the mapping between fixed ground locations and satellite beams constantly changes.
Davis, U.S. Pat. No. 5,239,668, incorporated herein by reference, discloses a satellite signalling system employing a synchronous satellite whose position relative to the earth remains fixed during transmissions. The synchronous satellite includes a steerable antenna which transmits a beam containing transmissions received from an earth-bound station to pagers which are positioned at various points on the earth's surface. As the beam moves from one location to the next, synchronization signals for different groups of pagers in a location covered by the beam are transmitted to the earth. Once synchronized, the pagers can adjust their power consumption circuitry to conserve power.
One conventional method of synchronizing hand-held communication devices employs an indirect addressing scheme in which the scheduling of transmissions is performed according to a particular beam number. Indirect addressing is addressing in which a hand-held communication device listens to transmissions in the beam currently positioned over the device to find out the time intervals during which the device must wake in order to receive future transmissions.
The disadvantage of the conventional method is the lack of robustness associated with indirect addressing schemes. If a user's hand-held communication device does not know the time interval during which it must wake to receive a current transmission, then it will not be able to tell in which time slot it must wake to receive future transmissions.